Luminescent Metal–Organic Framework with Outstanding “Turn-On” Hg2+ Sensing Ability First Constructed by an AIE Ligand

Hg2+ is highly toxic and can cause serious harm to the environment and humans. Thus, it is vital to develop efficient Hg2+ sensors. In this work, a LMOF-based (LMOF = luminescent metal–organic framework) "turn-on" Hg2+ sensor (1) is first developed by an aggregation-induced emission (AIE) functional ligand. 1 with a formula of [Cd3(L)2(NDA)3(H2O)2·(L)(MeOH)]n (L = 4,4′-((2,7-di-tert-butyl-9H-fluoren-9-ylidene)-methylene)dipyridine; H2NDA = 1,4-naphthalenedicarboxylic acid) exhibits a unique (3,4)-connected 2-D framework with 3-connected Cd1, 4-connected [Cd2(COO)3]+ cluster, and L and NDA2– bridges. The fluorene rings of L in 1 stack in a back-to-back manner. 1 has excellent water, pH, and thermal stabilities and can "turn-on" detect Hg2+ with ultrahigh sensitivity, good selectivity, and recyclability in a water medium. Furthermore, 1 shows obvious naked-eye-visible emission change and an extremely fast sensing response toward Hg2+ (<1 S) and can maintain its enhanced intensity over 5 min. The test paper is developed for practical "turn-on" Hg2+ sensing application. The "turn-on" Hg2+ sensing mechanism of 1 is discussed in detail.